A. Field of the Invention
The device of the present invention generally relates to the art of suspension insulators and more particularly to a new and improved suspension insulator for very high voltage application that is designed to provide more reliable performance over a wide temperature range.
B. Description of the Prior Art
Suspension insulators for supporting high voltage power cables suspended from high transmission towers are old and well known in the art. The above-identified co-pending application Ser. No. 576,731 discloses and claims a new and improved suspension insulator and more particularly a new and improved end connector for a suspension insulator. The end connector disclosed in the above-identified copending application includes an epoxy resin compound filling the space between inclined metallic surfaces of the end connector and an elongated rod used to support a load in tension. Where a suspension insulator is subjected to a very wide temperature range and/or for very high voltage applications where external flashovers may generate high current flows and the resultant large amounts of heat, the mechanical connection between the end connector and the rod through the epoxy resin compound may weaken due to the heating of the epoxy resin compound. Ultimately, the weakening in the bond may result in the separation of the end connector from the rod and the resultant separation of the high voltage power cable from the transmission tower.
The suspension insulator as disclosed in the aforementioned co-pending application includes an end connector arrangement affixed to an elongated rod used to support a load in tension. A plurality of insulating shells are serially disposed along the rod and between the end connectors. The elongated rod for example is a resin bonded, glass reinforced type and the ceramic insulating shells typically are formed from porcelain each having an elongated central bore and an integrally formed radially, outwardly extending skirt or shed. The space between the central bore of each of the shells and the outer periphery of the elongated rod is filled with elastomeric filler material.
When suspension insulators of this general type are exposed to wide ranges of temperature extremes, the coefficient of thermal expansions for the various materials used over these wide temperature ranges can lead to catastrophic failure modes of the suspension insulator and tend to destroy the overall suspension insulator assembly. In part, this is caused by the different coefficients of thermal expansion between the materials and at their interfaces; for example at the various interfaces between the insulating shells, the elongated rod and the filler material between the shells and the rod. In a particular example, the porcelain material of the insulating shells has an expansion coefficient of 2 ppm/.degree.F. and the filler material has an expansion coefficient of 35 ppm/.degree.F. Further, the material of the elongated rod has an expansion coefficient of approximately 2.5 ppm/.degree.F. in the longitudinal direction and approximately 20 ppm/.degree.F. in the radial direction. Thus, it is apparent that the filler material expands more than its interfacing restraining surfaces, namely the procelain insulating shells and the elongated, glass reinforced rod.
Further, the volumetric changes experienced by the various component parts of the suspension insulator due to thermal expansion and contractions of the elements also increases hermetic sealing problems that allow the ingress of moisture and contaminants leading to potential break down conditions and failure of the suspension insulator.
In the aforementioned co-pending application Ser. No. 576,731 and in U.S. Pat. No. 3,549,791 which issued to E. H. Yonkers on Dec. 22, 1970, there is disclosed an arrangement for maintaining the integrity of the interfaces between the elongated rod and the filler and also the filler and the insulating shells under various operating conditions by compression loading of the shells and the filler material. The compression loading is obtained by providing springs acting between the end fittings and the shells and the filler material.
British Patent Nos. 983,526, to Milligan et al and 878,073 to Proud et al disclose electrical insulator arrangements wherein flexible washers are disposed to separate tubular insulating bodies to maintain a water-tight seal during flexing.
While the above-described arrangements are generally suitable for their intended use, the suspension insulators of the prior art do not provide expansion compensation for a suspension insulator due to the differences of thermally induced volumetric changes of the insulating shells, the rod and the filling material without the use of compression loading techniques or springs.